This invention relates generally to gear pumps or motors, and more particularly to improvements in such devices of the type using "Gerotor" mechanisms wherein externally and internally toothed members cooperate to define successively expanding and contracting fluid compartments during rotation of the members on spaced parallel axes, the teeth of the members moving into and out of intermeshing engagement. Fluid flows into the expanding compartments and out of the contracting compartments through inlet and outlet ports respectively in at least one of the end walls of the cylindrical chamber defined by a casing and in which the toothed members are rotatably mounted. Heretofore, the inlet and outlet ports have been more or less kidney shaped, having square or rounded ends and, in many instances, the expanding compartments move beyond the end of the inlet port before becoming completely filled with fluid, thus contributing to inefficient operation. When such a device is used with incompressible liquids, such as water or oil, noisy operation and undue wear occurs.